The major event leading to the activation of a skeletal muscle have been worked out, but there are further details that are important to know if one is to understand this system fully. The aims of the present proposal are to study some of the excitation-contraction (e-c) coupling steps not yet well-defined, and some mechanisms by which they are controlled: I. - Now that we have found that mechanical fatigue is associated with a lack of activation of groups of myofibrils, we would like to investigate the mechanisms responsible for this failure. All the myofibrils in fatigued fibers can be activated with caffeine, therefore, the primary cause of failure lies in earlier steps of e-c coupling: either the tubular action potential (TAP) fails to propagate all along the T system or the T system-TC signal is altered in some myofibrils. Therefore, we will investigate: a) the TAP spread through the entire tubular network and the extent of the delay between T-system and Ca release from the TC; b) the amount of Ca released by the TC in local regions to see if the failure is due to a block in TC release of Ca; c) the passive electrical properties of the fatigued cells. We would like to know if the same mechanisms leading to fatigue operate in both amphibian and mammalian muscle. II. - The second series of experiments is to determine how the intracellular pH changes during fatigue and if it is related to the myofibrilar inactivation. We will determine the pHi in the micro-environment surrounding the myofibrils and relate this with tension and inactivation of the myofibrils. III. - After we have defined the release by and return of Ca to the TC, we will determine whether the voltage dependent recovery of contractility (repriming) after a prolonged depolarization with K+ is a function of the return of Ca to the TC or if it is associated with release of Ca.